Disk drives are commonly used in workstations, personal computers, laptops and other computer systems to store large amounts of data in a form that can be made readily available to a user. In general, a disk drive comprises a magnetic disk that is rotated by a spindle motor. The surface of the disk is divided into a series of data tracks. The data tracks are spaced radially from one another across a band having an inner diameter and an outer diameter. Each of the data tracks extends generally circumferentially around the disk and can store data in the form of magnetic transitions within the radial extent of the track on the disk surface. Typically, each data track is divided into a number of data sectors that store fixed sized data blocks.
A head includes an interactive element, such as a magnetic transducer, that is used to sense the magnetic transitions to read data, or to conduct an electrical signal that causes a magnetic transition on the disk surface, to write data.
The magnetic transducer includes a read/write gap that positions the active elements of the transducer at a position suitable for interaction with the magnetic transitions on the surface of the disk, as the disk rotates.
In accordance with known disk drive design, the head is electrically coupled to a pre-amplifier. During a read operation, electrical signals transduced by the transducer from the magnetic transitions on the disk surface, are processed by the pre-amplifier and transmitted to a read/write channel in the disk drive for eventual transmission to a host computer using the disk drive to store data. During a write operation, electrical signals representative of data are received by the read/write channel from the host computer for transmission to the preamplifier. The pre-amplifier includes a write driver electrically coupled to the head transducer to transmit the signals corresponding to the data to the head. The head is responsive to the signals received from the write driver to conduct a current and thereby cause magnetic transitions on the disk surface corresponding to the data.
As known in the art, the magnetic transducer is mounted by the head to a rotary actuator arm and is selectively positioned by the actuator arm over a preselected data track of the disk to either read data from or write data to the preselected data track of the disk, as the disk rotates below the transducer. The head structure includes a slider having an air bearing surface that causes the transducer to fly above the data tracks of the disk surface due to fluid currents caused by rotation of the disk.
In modern high capacity disk drives, the spindle motor is arranged to mount a stack of axially aligned storage disks, with the storage disks in the stack being spaced from one another. The use of multiple disks increases the total disk surface available for the storage of data. A head stack assembly comprises a stack of actuator arms, each mounting a head or a pair of heads. The stack of actuator arms is arranged adjacent the slack of storage disks with each head being positioned by the respective actuator arm over the surface of a corresponding one of the disks.
Two aspects of conventional disk drive design are position control of the heads and address headers for the data sectors recorded in the data tracks. The position control is used to accurately position a head over a data track for data read or write operations. Address headers are used to provide unique identification information for data stored in a particular data sector.
Whenever data are either written to or read from a particular data track, the transducer gap of the corresponding head must be centered over the centerline of the magnetic transitions of the data track where the data are to be written or from where the data are to be read, to assure accurate transduction of the transitions representing data. If the head is off-center, the head may transduce transitions from an adjacent track.
A servo system is typically used to control the position of the actuator arm to insure that the head is properly centered over the magnetic transitions during either a read or write operation. In a known servo system, servo position information is recorded on the disk surface itself, and periodically read by the head for use in controlling the position of the actuator arm. Such a servo arrangement is referred to as an embedded servo system. In modern disk drive architectures utilizing an embedded servo, each data track is divided into a number of data sectors for storing fixed sized data blocks, one per sector, as noted above. In addition, associated with the data sectors are a series of servo sectors that are generally equally spaced around the circumference of the data track. The servo sectors can be arranged between data sectors or arranged independently of the data sectors such that the servo sectors split data fields of the data sectors, as is well known.
Each servo sector contains magnetic transitions that are arranged relative to a track centerline such that signals derived from the transitions can be used to determine head position. For example, the servo information can comprise two separate bursts of magnetic transitions, one recorded on one side of the track centerline and the other recorded on the opposite side of the track centerline. Whenever a head is over a servo sector, the head reads each of the servo bursts and the signals resulting from the transduction of the bursts are transmitted to, e.g., a microprocessor within the disk drive for processing.
When the head is properly positioned over a track centerline, the head will straddle the two bursts, and the strength of the combined signals transduced form the burst on one side of the track centerline will equal the strength of the combined signals transduced form the burst on the other side of the track centerline. The microprocessor can be used to subtract one burst value form the other each time a servo sector is read by the head. When the result is zero, the microprocessor will know that the two signals are equal, indicating that the head is properly positioned.
If the result is other than zero, then one signal is stronger than the other, indicating that the head is displaced from the track centerline and overlying one of the bursts more than the other. The magnitude and sign of the subtraction result can be used by the microprocessor to determine the direction and distance the head is displaced from the track centerline, and generate a control signal to move the actuator back towards the centerline.
In a conventional disk drive design, each data sector of a data track is divided into a number of fields, including an address header field that contains magnetic transitions representing unique identification information for the specific data stored in the data sector. In this manner, the disk drive system can locate and verify the exact data sector for any particular block of data that a host computer may require, e.g., in a read operation. Among the information stored in an address header field is head identification information to uniquely identify the particular head of the head stack assembly that is transducing the magnetic transitions. During certain types of disk drive failures or error conditions, the electronics system in the disk drive is unable to identify which particular head is transducing magnetic transitions. The head identification information can then be read by the active head and used to determine which disk surface is being read.
Overhead refers to portions of a disk surface that are used to store information necessary for the control of the disk drive. Space on a disk surface used to store control information is not available to store data, and thus reduces the storage capacity of the disk drive. The servo sectors and address headers discussed above are examples of overhead. One proposal for increasing the storage capacity of a disk drive is referred to as a headerless format. In a headerless format, the headers are removed from the data fields to reduce overhead and thereby free up additional space on the disk surfaces that can then be used to store data. The headers are stored in RAM memory available in the disk drive electronics system. Careful monitoring of clock signals is relied upon to associate the data fields on the disk surface with the complementary headers containing the unique identification information.
In a headerless format, there is a risk that data cannot be located. For example, during a failure condition of the type discussed above, the electronics system of the disk drive would not be able to identify which head is active, and the lack of headers recorded on the disk surface leaves the electronics system without a source of unique identification information. A solution to this problem is to record the head identification information portion of the header within the servo sectors.
One step in the process of manufacturing a disk drive is a servo writing operation when first installed, the disks are blank, and the servo writing operation involves the performance of a series of writes to all the disk surfaces to record the servo sectors. The most efficient form of servo writing involves simultaneous activation of the heads of the head stack assembly for parallel writing of servo patterns. Since the disks are stacked by the spindle motor in an axially aligned arrangement, the data tracks of the disks, as defined by servo position information in the servo sectors, can be aligned with one another. Ordinarily, the servo pattern used for any particular data track can be common in content to all servo sectors of a set of data tracks that are axially aligned with the particular data track. Thus, the heads of the head stack assembly can be activated in parallel, via an external pre-amp parallel write or an internal multi-head write, to simultaneously record servo patterns on the aligned set of data tracks, using common servo information input from a single servo write channel.
However, in the headerless format described above, each head must record, in addition to the common servo pattern, head identification information that is unique to that head. Accordingly, a conventional parallel write is not feasible. A serial or staggered write does permit each head to record unique identification information, but results in a significant increase in servo write time since the servo sectors would be recorded one head at a time. Such an increase in servo write time can be seriously detrimental to a commercially viable disk drive mass production operation. Thus, there is a need for a servo write system that implements a parallel write operation for maximum efficiency in a disk drive manufacturing operation, while permitting the writing of unique identification information by each head.